FIG. 4 is a sectional side view of a conventional fuel supply apparatus disclosed in Int'l. Pat. App. Pub. No. WO96-23967.
In the figure, a fuel supply apparatus 1 integrally comprises a cover 3, a fuel pump 4, a fuel filter 5, a fuel pressure regulator 6, a discharge pipe 7, and a fuel level gauge and an electrical connector that are not illustrated in the figure, and the apparatus is suspended from an opening 2a of a fuel tank 2 formed of metal or resin.
The fuel filter 5 comprises a holding member 5a, formed of electrically conductive resin and a filter element, 5b contained therein, and is welded, liquid-tight, at its boundary portion 8 to the cover 3. The holding member 5a in its center portion holds the fuel pump 4, and at its lower end holds the fuel pressure regulator 6 and the fuel level gauge which is not illustrated. A gasket 9 is interposed to maintain an airtight seal between the cover 3 and the fuel tank 2.
In the holding member 5a of the fuel filter 5, an inlet 10 as a fuel inlet for the fuel filter 5 is provided in the upper inside periphery of the holding member 5a, and is connected via a sealing member 10a to a discharge pipe 11 of the fuel pump 4.
Moreover, the holding member 5a of the fuel filter 5 has two fuel outlets. A channel 12 (indicated by the dotted-dashed line), which forms the first fuel outlet, extends from the lower end of the holding member 5a axially in an upward direction. The channel 12 connects with the discharge pipe 7 provided on the cover 3 and supplies fuel, that has been filtered, to an injector. A return path 13, forming the second fuel outlet, is provided in the lower portion of the holding member 5a, and is connected to the fuel pressure regulator 6.
Reference numeral 14 is a pump filter, and when the fuel in the fuel tank 2 is drawn into the fuel pump 4, the pump filter 14 filters out foreign particles from the fuel tank 2, such as iron powder, etc.
The fuel pressure regulator 6 is fixed by inserting a base 6a into an insertion member 5c that extends downward from the bottom portion of the holding member 5a. A diaphragm 6c is interposed between an opening end of the base 6a and a housing 6b, and the diaphragm 6c is supported by a movable valve seat 6e provided with a discharge orifice 6d. 
A stationary valve seat 6f that co-functions with the movable valve seat 6e is fixed inside the base 6a. A spring 6g is contained between the housing 6b and the diaphragm 6c, and, at the lower portion of the housing 6b, a discharge outlet 6h is disposed to discharge the fuel from the discharge orifice 6d into the fuel tank 2.
Reference numeral 4a is an electric current supply unit for supplying current to a motor, not illustrated, for the fuel pump 4, and the electric current supply unit 4a is connected electrically to the electrical connector, not illustrated, which is formed integrally with the cover 3.
In the fuel supply apparatus 1 configured in this way, electric current is supplied from a power supply, such as a storage battery, etc., not illustrated, via the electric current supply unit 4a, to the motor, not illustrated, of the fuel pump 4. When the motor runs, the fuel in the fuel tank 2 is taken in through the pump filter 14, and after being discharged from the discharge pipe 11, it flows through a channel 15 in the direction of arrow A, then passes through the filter element 5b, and arrives at a lower space 16 under the filer element 5b. 
Next, the fuel flows into the fuel pressure regulator 6, and when fuel pressure in the base 6a becomes higher than the set pressure of the spring 6g, the diaphragm 6c moves to the housing 6b side, opening the discharge orifice 6d of the movable valve seat 6e and discharging the fuel in the base 6a to the fuel tank 2 again (returning flow volume) via the discharge outlet 6h, and the remaining volume flows through the channel 12 in the direction of arrow B and is supplied at a predetermined pressure through the discharge pipe 7 to the injector, in a fuel injection device mounted on an engine, which is not illustrated.
FIG. 5 is a pressure characteristic diagram of the conventional fuel supply apparatus, illustrating the relation between the returning flow volume (Q) discharged from the fuel pressure regulator 6 to the fuel tank 2, and the fuel pressure (P) of the fuel supplied to the injector. When the returning flow volume (Q) is over Qm, the conventional fuel supply apparatus 1, configured as described above, ensures that the pressure of the fuel delivered under pressure to the injector is above a predetermined pressure Pi, and supplies the fuel to the injector of the fuel injection apparatus attached to the engine.
However, for example, if the voltage supplied to the fuel pump 4 drops abruptly and the discharging performance of the fuel pump 4 deteriorates, or if the fuel that is supplied to the engine abruptly increases, the returning flow volume drops (shifts to the left in the diagram), and thus the problem has occurred that the fuel delivered under pressure to the injector cannot be ensured at the predetermined pressure Pi, and the injector cannot inject the fuel required by the engine.
The present invention is made to solve the above-described problems, and it has as an object the provision of a fuel supply apparatus in which, if the returning flow volume (Q) drops, the pressure of the fuel delivered under pressure to the injector does not drop.